Power oscillator for control of waveshape and amplitude

ABSTRACT

An RF power oscillator for contactless card antennas shapes a carrier signal at the operating frequency utilizing a delay circuit having a number of taps for delaying the carrier signal by different lengths of time. The delayed signals are input into a buffer and output through resistors to a node coupled to the antenna. The resulting waveform for a square wave input signal, and equal-length delay taps, is a trapezoidal wave output. Any input wave form can be shaped in a variety of ways depending upon the combinations of delay taps used. Since the buffer drivers for each delayed wave switch state at slightly different times, the amplitude and bandwidth of emitted electromagnetic interference (EMI) is reduced for the transmission circuit.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S.provisional application Ser. No. 60/269,341 filed Feb. 16, 2001 which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] This invention relates generally to radio frequency (RF) poweroscillators for contactless card antennas, and more specifically to anRF power oscillator utilizing tapped delay lines and digital buffers forshaping the operating frequency input signal to minimize unwantedharmonics and reduce electromagnetic interference.

[0004] 2. Background

[0005] Smart card signal transmission circuitry includes at least oneoscillator circuit for generating a modulated carrier signal fortransmission of data to a smart card. A common class of output stageutilized for RF communication is Class-A output stages which is capableof generating pure sine waves due to its linear characteristics. Due tothe low efficiency of the Class-A output stages, non-linear power stagesor square wave generators are typically used in the prior art as theradio frequency (RF) power oscillators for contactless card antennas.However, these non-linear or square wave generators present severaldisadvantages for use in providing a modulated carrier signal fortransmission by smart card antennas.

[0006] A disadvantage of the prior art nonlinear transistor stage is thedependency on the transistors gain parameters. This type of output stageis typically based on a NPN transistor with a parallel LC resonancecircuit as collector load. This circuit is capable of generating fairlypure sine waves, but the nonlinear nature of the circuit makes it veryhard to control the amplitude of the output signal and especially themodulation index in case amplitude modulation is desired.

[0007] A disadvantage of the prior art square wave generator is that thegenerator draws a large current spike from the power supply when itswitches state due to the charging and discharging of inherentcapacitances in the switching circuit. The current spike typically has aduration comparable to the rise and fall-time of the output square wave,resulting in a current spike with a very broad electromagneticinterference (EMI) noise spectrum. The prior art circuits that use thenonlinear or square wave generator also require the use of a low pass ora band pass filter before the modulated signals are fed to the tunedantenna coil to rid the signal of the harmonics of the operatingfrequency. However, these filters include combinations of capacitors andinductors which produce additional signal interference between thefilter and the tuned antenna coil resulting in unwanted resonances atfrequencies outside the operating frequency of the smart cardcommunication system.

[0008] Therefore, a need continues to exist for a radio frequency poweroscillator for use with contactless smart card antennas that willproduce a high current, modulated signal with an improved wave shape andaccurately controlled amplitude without drawing excessive current spikesand with reduced electromagnetic interference.

SUMMARY OF THE INVENTION

[0009] It is an advantage of the present invention to provide a poweroscillator circuit for control of the wave-shape and the amplitude of anoutput data signal.

[0010] It is another advantage to provide a high current/low impedancemodulated output signal for use with a smart card antenna.

[0011] Still another advantage is to provide an RF power circuit havinglow electromagnetic interference.

[0012] It is yet another advantage that the amplitude and modulationindex is accurately controlled by the supply voltage of the outputstage.

[0013] In the exemplary embodiment of the present invention a poweroscillator circuit generates a wave-shaped and amplitude controlledoutput signal for transmission on a smart card antenna. The poweroscillator includes an on/off modulated carrier input signal connectedto a tapped delay line. Multiple tap outputs of the delay line areconnected to the inputs of a selected number of buffers. The outputs ofthe buffers are connected in series with same value resistors, and thebuffer output resistor lines are connected in parallel to a single node.The progressively delayed input signals on the buffer output resistorlines are hard-wire combined at the single node to produce a wave-shapedoutput signal. For a square wave carrier input signal having a 50% dutycycle, and a tapped delay line have equal-length delay taps, theresulting wave-shaped output signal is trapezoidal with a rise and falltime equal to the number of taps multiplied by the delay time betweentaps. In other embodiments of the invention, the power oscillator may beconfigured to generate a different output signal depending upon theconfigurations of delay taps used. Since the buffer drivers for eachdelayed output signal switch state at slightly different times, theamplitude and bandwidth of emitted electromagnetic interference (EMI) isreduced significantly.

[0014] The power oscillator of the present invention also offers theadvantage of control of the amplitude of the wave-shaped output signalfor amplitude modulation of less than 100%. The exemplary embodimentprovides 0-25% modulation utilizing a power supply circuit which outputsa desired transmission voltage. These modulation percentages are used inthe particular applications for smart card antennas as specified in theISO14443 standard. As an example, an ISO14443 type-B contactless smartcard requires a modulation index of 10%. This is achieved in the presentinvention by switching between a supply voltage of Vmean+10% andVmean−10%. If Vmean+10% is 5.0V, then Vmean equals 5V1.10, and Vmean−10%equals 0.90×(5V/1.10), or 4.09V. The transmission voltage generated bythe power supply circuit is connected to the power supply inputs of thebuffers. The buffers output signals are then limited to the voltageamplitude of the power input to the buffers resulting in the desired 10%modulation index amplitude modulation. The maximum modulation index islimited by the minimum operating voltage of the buffers.

[0015] In an exemplary method of the present invention for controllingthe wave shape and amplitude of a modulated carrier signal, themodulated carrier signal is produced utilizing a power oscillatorcircuit which includes readily available, low cost CMOS line drivers asthe RF power source. Each line driver is a 74AC541 driver manufacturedby Texas Instruments, or any other suitable line driver, which has eightindividual buffers. The exemplary embodiment utilizes a total of threeline drivers. Two of the buffers of the first line driver are used fordriving the delay line, and two of buffers of the third line driver areused for driving the termination of the delay line to either 2.5 v or0.0 v to conserve energy in idle mode. Therefore, there are twentybuffers available for connection to the twenty taps of the tapped delayline. A square wave signal at the operating frequency and with 50% dutycycle drives the inputs of the CMOS line drivers. If 100% AM modulationis required, the data signal input will be gated digitally, preferablysynchronized to the operating frequency. If 0 to 25% modulation isrequired, the supply voltage for the CMOS line drivers is modulatedaccordingly by the power supply circuit. The outputs of the CMOS linedrivers are connected in parallel with a 82 ohm resistor in series withthe output of the CMOS line drivers. This value is chosen in order tominimize the influence of variations in buffer output impedance. If thetypical output impedance of the buffer is 25 ohm with a tolerance of+/−50%, then the apparent output impedance tolerance of each buffer willbe reduced to +/-12% if 82 ohm 1% resistors are added to the output. Thevalue of the resistors for other embodiments may range from 22 ohms to100 ohms in accordance with the typical output impedance of the buffers.

[0016] The inputs of the CMOS line drivers are connected to the tappeddelay-line having equal length delays between the inputs of the CMOSline drivers. The signal will typically travel at a speed of less than200 mm/ns in a buried stripline. The length of the delay line betweeneach tap is approximately 112 mm. In the exemplary embodiment of thepresent invention, the tapped delay trace is a buried stripline on a 6layer printed circuit board. The stripline runs in layer 4, and layer 2and 6 are ground planes on each side of the stripline. The width of thestripline is approximately 0.2 mm, and spacing between each stripline isapproximately 0.2 mm. This results in an impedance of approximately 75ohms, and a delay of approximately 180 mm/ns. The resulting trapezoidalwave has a rise and fall time of approximately 12.5 ns.

[0017] The output of the twenty paralleled resistors is lowpass filteredwith a 1200 pF capacitor, C3, to ground. The resulting output impedanceof the power oscillator, at node 108,110 is approximately (25+82)/20ohm=5 ohm in parallel with 1200 pF, or approximately 3.3 ohm at 13.56MHz. This is sufficiently low for driving a parallel tuned antennathrough a capacitive network, without loading the Q factor of the tunedcircuit excessively. As the impedance of the node 108, 110 is very low,the tuned circuit C1, L1 effectively has C2 connected in parallel toground. C2 and C1 will typically have a value of 220 pF.

[0018] The point in time where each individual buffer switches isdistributed over a period equal to the resulting rise time of the outputwave form, resulting in a transient current draw from the power supplythat is distributed in time as well. If for example, twenty (20) buffersare used, the resulting transient current draw can be twenty (20) timeslower and spread over a twenty (20) times longer period, compared to asystem where all buffers switch at the same time. This reduces theamplitude and bandwidth of the resulting emitted EMI from the circuitconsiderably.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The present invention will be better understood from thefollowing detailed description of a preferred embodiment of theinvention, taken in conjunction with the accompanying drawings in whichlike reference numerals refer to like parts and in which:

[0020]FIG. 1 is an block diagram of the power circuitry for control ofsignal wave shape and amplitude of a preferred embodiment;

[0021]FIG. 2 is a block diagram of a voltage control circuit of apreferred embodiment;

[0022]FIG. 3 illustrates a waveform of a square wave input signal to afirst buffer along the tapped delay line;

[0023]FIG. 4 illustrates a waveform of the output signal of the firstbuffer;

[0024]FIG. 5 illustrates a waveform of a delayed square wave input to alast buffer along the tapped delay line;

[0025]FIG. 6 illustrate a waveform of the output of the last buffer;

[0026]FIG. 7 illustrates a waveform of the output signal of theparalleled buffers without capacitive/antenna loading;

[0027]FIG. 8 illustrates a waveform of the output signal of theparalleled buffers with capacitive loading from C3; and

[0028]FIG. 9 illustrates a waveform of the output signal on a tunedone-turn antenna coil.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0029] The following detailed description utilizes a number of acronymswhich are generally well known in the art. While definitions aretypically provided with the first instance of each acronym, forconvenience, Table 1 below provides a list of the acronyms andabbreviations and their respective definitions. ACRONYM DEFINITION ACalternating current AM amplitude modulation CMOS complementarymetal-oxide semiconductor RF radio frequency

[0030]FIG. 1 illustrates a RF power oscillator circuit 2 of a preferredembodiment for the control of the wave shape and the amplitude of acarrier signal 4. The RF power oscillator circuit 2 includes a carriersignal 4 coupled to a tapped delay line 22, line drivers 8, 10, 12having multiple inputs, a tap 54 for connection to the inputs of theline drivers 8, 10, 12, line driver enable circuitry Q1, R5, R4, linedriver output resistors 14, 26, 28 and an output capacitive network C1,C2, C3 coupled to an antenna coil 20. The operating voltages V_(CC) andV_(CC) transmit 18 are supplied by a supply voltage control circuit 40illustrated in FIG. 2.

[0031] Continuing with FIG. 1, the carrier signal 4 of the preferredembodiment is a square wave at the operating frequency, e.g. 13.56 MHz,with a 50% duty cycle. If 100% AM modulation is required, a data signalinput (not shown) will be gated digitally, preferably synchronized tothe operating frequency, to produce a modulated carrier signal 4. If 0to 25% modulation is required, a V_(CC) transmit voltage input 18 of theline drivers may be modulated accordingly by the supply voltage controlcircuit 40 of FIG. 2.

[0032] As shown in FIG. 1, The modulated carrier signal 4 is input tothe line driver 8 through resistors R1 and R2 which serve to limit theamplitude of the modulated carrier signal 4 in order not to exceed themaximum input voltage specifications of the line driver 16. The linedrivers 8, 10, 12 of the preferred embodiment are low cost and readilyavailable complementary metal-oxide semiconductor (CMOS) line driverssuch as the octal buffer/line drivers, part number 74AC541, manufacturedby Texas Instruments. The line drivers 8, 10, 12 typically are arrangedin packages of eight individual drivers. The outputs of the CMOS linedrivers 8, 10, 12 are connected in parallel to a single output node 50.In the preferred embodiment, each line driver output 102, 106 isconnected in series with a resistor 14 to limit the output current ofthe driver and control the output impedance. The resistors 14 of thepreferred embodiment are of equal resistance of 82 ohms to ensure thatthe driver outputs 102, 106 have the same electrical characteristics.For a 74AC541 buffer driver, the resistor values may range from 22 ohmsto 100 ohms. If the values is too low, the variances in output impedanceof the drivers becomes dominant, and if the value is too high, theoutput power of the circuit will be limited.

[0033] As shown in the preferred embodiment of FIG. 1, the modulatedinput signal 4 is coupled to the first two inputs 58 of a first buffer8. The corresponding buffer output lines are connected in parallelthrough series resistors 28 to a single node 52. The single node 52serves as the input to the tapped delay line 22. In the preferredembodiment, series resistors 28 have a value of 4.7 ohms to ensure thatthe load is evenly distributed between the two buffers. The bufferenable lines are connected to the enable circuitry 6, Q1, R5. Q1 acts asan inverter for the “TRANSMITTER ENABLE” signal.

[0034] The inputs 100 of the CMOS line drivers 8,10, 12 are connected tothe tapped delay-line 22. In a preferred embodiment of the invention,the individual delays between the inputs 100 of the CMOS line drivers8,10,12 are equal. This configuration results in an output signal 108signal having a trapezoidal wave shape. A more complex delaydistribution may be utilized to produce a desired wave shape, forexample, a sine wave shape. The tapped delay line 22 of the preferredembodiment is constructed using a stripline path embedded in a printedcircuit board with a distance L between each tap 54. In otherembodiments of the invention, a conventional delay line circuit may beused such as a delay IC or a LC delay line.

[0035] The delayed output signals 102 of the preferred embodiment ensurethat all of the buffers of the line drivers 8,10,12 switch at differentpoints in time over a period equal to the resulting rise time of theoutput wave form. The distributed switching of the buffers of the linedrivers 8, 10, 12 results in a transient current draw from the powersupply circuit 40 that is distributed in time as well. If, for example,twenty buffers are used for shaping the output waveform 108, theresulting transient current draw is twenty times lower and spread over atwenty times longer period as compared to a system where all buffersswitch at the same time. Thus, the RF power circuitry 2 of the preferredembodiment considerably reduces the amplitude and bandwidth of theresulting emitted EMI from the circuit 2.

[0036]FIG. 2 is an illustration of the supply voltage control circuit 40for amplitude modulation of 0% to 25%. The V_(CC) transmit voltageoutput 18 of this circuit may be controlled to provide a requiredamplitude of the output signal 108, 110. The power supply circuit of thepreferred embodiment includes a V_(CC) power supply of 5 volts connectedto the source of a P-channel field-effect transistor (FET) Q4. When thehigh value of the amplitude modulation is desired, the FET Q4 isswitched on, and when low value of the amplitude modulation is desired,the P-channel FET Q2 is switched on. Capacitors C4 and C5 are decouplingcapacitors for the 5V supply and C6 and C7 are decoupling capacitors forthe 3.5-5V supply. Typical values are 0.1 uF and 10 uF. Capacitor C8 isa decoupling capacitor for the Vccxmit node. The CMOS inverter 34ensures that the control signal 30 swings between 0 and 5V, and theinverter 32 inverts the signal 30, so that Q2 and Q4 are never switchedon at the same time. Resistors R6 and R8 limit the rise time of Q2 andQ4, respectively, so that the drain current is limited to safe valueswhen Q2 and Q4 switch state. FET Q2 is coupled as a source follower. Thevoltage at Vccsmit 18 will always be equal to or larger than the voltageat Vcc(3.5-5V) 38, so the intrinsic diode from drain to source in Q2will never conduct.

[0037] In a method for controlling wave shape and amplitude of ancarrier signal for transmission by a smart card antenna, an RF poweroscillator utilizes three 74AC541 line drivers 8, 10, 12 having eightbuffers each. Two of the line buffers of the first line driver 8 areused for driving the tapped delay line 22. The last two buffers of thethird line driver 12 are used by the enable circuitry 6, R4 for drivingthe termination of the delay line to either 2.5 v or 0.0 v to conserveenergy in idle mode. The tapped delay line 22 uses the remaining twentytaps for shaping the output waveform 108.

[0038] The length L of the delay line between each tap is approximately112 mm. The traces of the tapped delay line 22 are implemented as aburied stripline on a layered printed circuit board (not shown). Thestripline is placed in an inner layer and is located between two groundplane layers. The width of the stripline is approximately 0.2 mm and thespacing between each stripline is approximately 0.2 mm. Thisconfiguration of the stripline has a line impedance of approximately 75ohm with a delay of approximately 0.6 ns between taps.

[0039]FIGS. 3 through 9 illustrate the input and output signals for theRF power circuit 2 of the preferred method for controlling the waveshape and amplitude of a carrier signal. FIG. 3 is an illustration of asquare wave input signal 100 at the first tap of the tapped delay line22. FIG. 4 illustrates the first buffer output 102 of the first tap ofthe tapped delay line 22. The first buffer output 102 is delayed due tothe input-to-output delay of the line driver 8. FIG. 5 is anillustration of the square wave input signal 104 at the last tap of thetapped delay line 22, and FIG. 6 is an illustration of the last bufferoutput 106 of the last tap of the tapped delay line 22. A comparison ofFIG. 4 and FIG. 6 demonstrates that the last buffer output signal 106 isdelayed by approximately 12.5 ns from the first buffer output signal102. The resulting trapezoidal signal 108, shown in FIG. 7 and measuredat test point 24 of FIG. 1, has a rise and fall time of approximately12.5 ns.

[0040] When capacitive loading C3 is added to the output node 50 of theRF power circuit 2, the filtered output 110 of FIG. 8 is produced. Inthe preferred embodiment, the capacitive loading includes a 1200 pFcapacitor C3 to ground, and a 220 pF capacitor C2 in series withparallel resonance circuit consisting of the inductance L1 of the coilantenna 20 and capacitor C1. The output impedance of the power circuit 2that is driving the capacitive network that powers antenna 20 isapproximately 5 ohm in parallel with 1200 pF, or approximately 3.3 ohmat the 13.56 MHz operating frequency of the output carrier signal 110.This is sufficiently low for driving a parallel tuned antenna 20 throughthe capacitive circuit C2, C3 without excessively loading the Q factorof the power circuit 2. FIG. 9 illustrates the output signal 112 tunedto a sine wave utilizing the variable capacitor, C1 shown in FIG. 1.

[0041] Although a preferred embodiment of the invention has beendescribed above by way of example only, it will be understood by thoseskilled in the field that modifications may be made to the disclosedembodiment without departing from the scope of the invention, which isdefined by the appended claims.

I claim:
 1. A power oscillator for controlling the wave shape andamplitude of an input signal to produce a desired output signal, thecircuit comprising: a tapped delay line connected to the input signal,the tapped delay line having a plurality of taps, each tap separatedfrom an adjacent tap to produce a plurality of delayed input signals; atleast one buffer, the buffer comprising: a plurality of input linesconnected to the plurality of taps; an input voltage line for connectionto a controllable voltage source; and a plurality of output lines; and aplurality of resistors having first ends connected to the plurality ofoutput lines, the plurality of resistors having second ends connected inparallel to an output node to produce the desired output signal.
 2. Thepower oscillator of claim 1, wherein the buffer is a complementarymetal-oxide semiconductor (CMOS).
 3. The power oscillator of claim 1,wherein the desired output signal is coupled to an antenna coil of asmart card.
 4. The power oscillator of claim 3, wherein the desiredoutput signal is coupled through a capacitor circuit to the antennacoil, the capacitor circuit comprising: at least one capacitor forproviding capacitive loading; and an adjustable capacitor for tuning thedesired output signal.
 5. The power oscillator of claim 1, where in theinput signal is 100% amplitude (on/off) modulated.
 6. The poweroscillator of claim 5, wherein the input signal is a square wave carriersignal.
 7. The power oscillator of claim 1, wherein the tapped delayline comprises a stripline embedded in a circuit board.
 8. The poweroscillator of claim 1, wherein the controllable voltage source limitsthe operating voltage of the buffer to produce an amplitude modulationless than 100%.
 9. A method for shaping and controlling the amplitude ofan carrier signal, the method comprising the steps of: inputting thecarrier signal into a tapped delay line having a plurality of tapsseparated by a plurality of delay distances; connecting each tap of theplurality of taps to a buffer of a plurality of buffers; connecting aseries resistor to an output of each buffer of the plurality of buffers;connecting the outputs and series resistors of each buffer in parallelto a single node to produce a shaped carrier signal; and supplying asupply voltage to the buffers to control an output voltage amplitude ofthe shaped carrier signal.
 10. The method of claim 9, wherein theplurality of delay distances are equal.
 11. The method of claim 10,wherein the carrier signal is a square wave having a 50% duty cycle, andwherein the shaped carrier signal is trapezoidal.
 12. The method ofclaim 9, wherein the series resistor connected at the output of eachbuffer is 82 ohms.
 13. The method of claim 9, further comprising thestep of adding capacitive loading to the single node.
 14. The method ofclaim 13, further comprising the step feeding the shaped carrier signalto an antenna tuned to the operating frequency of the carrier signal.15. The method of claim 9, wherein the plurality of buffers arecontained in CMOS line driver packages.
 16. The method of claim 9,wherein the step of supplying a supply voltage to the buffers furthercomprises the steps of modulating the voltage of the power supplycircuit resulting in a directly proportional modulation of the antennafield in accordance with the desired percent modulation.
 17. The methodof claim 16, wherein the desired percent modulation is between 0 and 25%modulation and the supply voltage is at a level of 5 volts.
 18. Themethod of claim 16, wherein the desired percent modulation is between 0and 25% modulation and the supply voltage is at a level of 3.3 volts,using CMOS buffers operating at 3.3V.
 19. The method of claim 9, whereinthe plurality of delay distances are not equal, and the resulting shapedcarrier signal is sinusoidal.